1. Field of the Invention
This invention relates in general to refrigeration circuits and more particularly to a defrost system for use in a refrigeration circuit such as may be incorporated in air conditioning apparatus including a heat pump.
2. Prior Art
The conventional refrigeration circuit employs a compressor, condenser, expansion means and evaporator connected to form a refrigerant flow circuit. The compressor raises the pressure and temperature of gaseous refrigerant and the gaseous refrigerant is then conducted to the condenser wherein it gives off heat to a cooling fluid and is condensed to a liquid. This liquid refrigerant then flows through an expansion means such that its pressure is reduced and it is therefore capable of changing from a liquid to a gas, absorbing heat during the change in state. A complete change of state from a liquid to a gas occurs in the evaporator and heat is removed from the media flowing in heat transfer relation with the evaporator. The gaseous refrigerant from the evaporator is then conducted back to the compressor.
Under appropriate ambient conditions the media flowing in heat transfer relation with the evaporator, typically air, has its temperature lowered below its dew point. Once the temperature of the air is below the dew point, moisture is deposited on the coil surfaces resulting in a collection of fluid thereon, or if the ambient conditions are sufficiently low or the temperature of the evaporator is sufficiently low, ice is formed thereon. Once this ice or frost coats the surfaces of the heat exchanger the efficiency of the heat exchanger is impaired and overall system efficiency decreases. Consequently, it is desirable to maintain the evaporator surfaces free from ice or frost.
The formation of ice or frost on the heat exchanger surface is particularly acute with heat pumps used to provide heating to an enclosure. In operation of the heat pump in the heating mode the outdoor coil functions as an evaporator such that heat may be absorbed from the outside air. If the outside air is at a low temperature the evaporator must operate at an even lower temperature and consequently it may operate under the appropriate environmental conditions such that ice and frost are formed thereon.
Many systems have been developed for defrosting heat exchanger coils. These include supplying electric resistance heat to the coil surface to melt the ice and reversing a refrigeration system such that hot gas discharged from the compressor is circulated through the evaporator to melt the ice thereon. An inconvenience accompanying the reversing system is that heat is removed from the enclosure being heated during defrost.
Many non-reverse defrost systems i.e. systems which do not involve a reversal in the flow path of refrigerant through the refrigeration circuit, have been previously utilized and are disclosed in the art. Most of these systems however do not involve using liquid refrigerant to melt the ice on the heat exchanger in question. Most of these systems concern bypassing the condenser such that hot gas from the compressor is discharged directly into the evaporator and then some method is used to vaporize the refrigerant which is liquified in the exchanger or to maintain superheat in the refrigerant so that it never changes from a gas to a liquid. Herein the liquid refrigerant is subcooled in the evaporator such that the amount of heat given off to the heat exchanger to melt the ice formed thereon is a function of the temperature difference between the fluid entering the heat exchanger and the fluid being discharged from the heat exchanger.
In the present non-reversible hot gas defrost system superheated gaseous refrigerant is conducted in the defrost mode of operation to a heat exchanger mounted within an accumulator disposed between the evaporator and the compressor such that the hot gas discharged from the compressor circulates through the heat exchanger and then to the condenser. In the heat exchanger gaseous refrigerant changes state to a liquid and then is conducted through the condenser which is rendered inoperative for heat transfer to the evaporator where the liquid is further condensed and subcooled, melting the ice formed thereon. The liquid refrigerant being discharged from the evaporator is then conducted to the accumulator which partially fills with liquid refrigerant. The compressor suction line runs into the top portion of the accumulator such that gaseous refrigerant may be withdrawn therethrough while leaving liquid refrigerant within the accumulator. By supplying hot gas to the accumulator heat exchanger during defrost, heat energy is provided to vaporize the liquid refrigerant within the accumulator. By a combination of the vaporization of the liquid refrigerant and the decrease in pressure created by the suction of the compressor sufficient gaseous refrigerant is provided to supply the system. The condensing subcooling effect of the liquid refrigerant within the evaporator provides sufficient heat to melt the frost or ice formed on the surfaces of the heat exchanger.